Three-Dimensional
and Time-Ordered Surface-Enhanced
Raman Scattering Hotspot Matrix
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Abstract
The “fixed” or “flexible”
design of
plasmonic hotspots is a frontier area of research in the field of
surface-enhanced Raman scattering (SERS). Most reported SERS hotspots
have been shown to exist in zero-dimensional point-like, one-dimensional
linear, or two-dimensional planar geometries. Here, we demonstrate
a novel three-dimensional (3D) hotspot matrix that can hold hotspots
between every two adjacent particles in 3D space, simply achieved
by evaporating a droplet of citrate-Ag sols on a fluorosilylated silicon
wafer. In situ synchrotron-radiation small-angle X-ray scattering
(SR-SAXS), combined with dark-field microscopy and in situ micro-UV,
was employed to explore the evolution of the 3D geometry and plasmonic
properties of Ag nanoparticles in a single droplet. In such a droplet,
there is a distinct 3D geometry with minimal polydispersity of particle
size and maximal uniformity of interparticle distance, significantly
different from the dry state. According to theoretical simulations,
the liquid adhesive force promotes a closely packed assembly of particles,
and the interparticle distance is not fixed but can be balanced in
a small range by the interplay of the van der Waals attraction and
electrostatic repulsion experienced by a particle. The “trapping
well” for immobilizing particles in 3D space can result in
a large number of hotspots in a 3D geometry. Both theoretical and
experimental results demonstrate that the 3D hotspots are predictable
and time-ordered in the absence of any sample manipulation. Use of
the matrix not only produces giant Raman enhancement at least 2 orders
of magnitude larger than that of dried substrates, but also provides
the structural basis for trapping molecules. Even a single molecule
of resonant dye can generate a large SERS signal. With a portable
Raman spectrometer, the detection capability is also greatly improved
for various analytes with different natures, including pesticides
and drugs. This 3D hotspot matrix overcomes the long-standing limitations
of SERS for the ultrasensitive characterization of various substrates
and analytes and promises to transform SERS into a practical analytical
technique